Apparatus for use in well abandonment

ABSTRACT

A plug for plugging wells, and in particular oil and gas wells, is provided. The plug has a plug body formed from an outer metal tube of a reduced thickness. The plug also has reinforcement means, attached to an inner surface of the outer tube, that give the plug a cross-sectional structural strength that is at least equivalent to that of a thicker metal tube. The plug has a central heater receiving void located along the axis of the plug to enable a plug deployment heater to be received therein. Also provided is a plug assembly with a variable cross-sectional area in a plane perpendicular to the plane in which the assembly is deployed during the plugging of underground conduits.

RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 14/654,423, filed Jun. 19, 2015, which is a national stage entryunder 35 U.S.C. 371 of International Application No. PCT/GB2013/053397,filed Dec. 20, 2013, which claims priority to Great Britain ApplicationNo. 1223055.3, filed Dec. 20, 2012, each of which are each incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to the plugging of underground conduitssuch as wells, and in particular oil and gas wells. More particularlythe present invention relates to Bismuth-based or other eutectic plugsand plug deployment apparatus for use in the plugging of undergroundconduits such as wells.

BACKGROUND OF THE INVENTION

When a well, such as an oil or gas well, is at the end of its usefullife it is usually abandoned. In other situations (e.g. due to low oilprices, “workovers” and other events such as approaching storms orhurricanes) a well may need to be temporarily suspended. However beforea well can be abandoned/suspended the well must be “plugged” to ensurethat potentially hazardous materials, such as hydrocarbons, cannotescape the well.

In the past various methods have been employed to plug abandoned wells.One such known method involves pouring cement or resin into a well so asto fill a length of the well. However the use of cement/resin has provento be unreliable and vulnerable to leaking. This can lead to previouslyabandoned wells being re-plugged at considerable extra expense.

In view of the limitations of using cement/resin to plug wells analternative approach was developed which uses a bismuth-containing alloyto form a seal within the well. This approach, which is described indetail in CA 2592556 and U.S. Pat. No. 6,923,263, makes use of the factthat such alloys contract upon melting and expand again when theyre-solidify. Essentially the alloy is deployed into a well; heated untilit melts and “slumps”; and then allowed to cool whereby the alloyexpands to form a tight seal with the walls of the well.

The use of eutectic alloys, such as bismuth-containing alloys, to plugwells or repair existing plugs in wells is described in: U.S. Pat. No.7,290,609; 7,152,657; US 2006/0144591; U.S. Pat. No. 6,828,531;6,664,522; 6,474,414; and US 2005/0109511.

SUMMARY OF THE INVENTION

The present invention relates to improvements in the apparatus used in,amongst other things, well abandonment and well suspension. Theimprovements relate in particular to the plugs used to close off or‘plug’ wells (and other underground conduits), and the heaters used todeploy plugs and/or retrieve plugs from an underground conduit, such asa well casing.

A first aspect of the present invention relates to a plug for pluggingwells, and in particular oil and gas wells, said plug comprising a plugbody formed from an outer metal tube of a reduced thickness andreinforcement means, attached to an inner surface of the outer tube togive the plug a cross-sectional structural strength that is at leastequivalent to that of a thicker metal tube; and wherein said plug has acentral heater receiving void located along the axis of the plug.

By forming the plug from an outer tube that is thinner than typicalplugs and then reinforcing the outer tube in the cross-sectionaldirection (i.e. across the diameter of the plug) it is possible toprovide a plug that has all the required strength to maintain a plugwithin a well whilst at the same time allowing for the plug to be moreeasily drilled out (i.e. due to the weaker structural strength along thelength of the plug) if required.

By way of reference it should be noted that the wall thickness of thetubes currently being used in well plugs is in the region of 5 mm,whereas the wall thickness of the tubes used in the present invention isconsiderably less at around 1-3 mm.

Preferably the reinforcement means attached to the outer tube comprisescorrugated metal. Alternatively the reinforcement means attached toouter comprises a ‘honeycomb’ metal mesh. It is appreciated that boththese reinforcement means, when correctly orientated, achieve therequired structural strength in cross-section and yet remain weakeralong the length of the plug. This allows for the plug to be more easilydrilled out (i.e. because there is less metal to drill through).

Preferably the plug comprises a plug body formed from an outer metaltube and an inner metal tube connected together co-axially by way ofreinforcement means that attach to an inner surface of the outer tubeand the outer surface of the inner tube at discrete points.

Once again, by forming the plug body from two thinner metal tubesconnected together in this way it is possible to create a plug that isstrong in a horizontal plane (i.e. across the diameter of the tubes) butweaker in a vertical plane (i.e. down the co-axis of the tubes).

This selective weakness means that the plug body will perform itsfunction of plugging a well (for example) until such time as removal ofthe plug is required. If, in such situations, the plug cannot beretrieved using the apparatus and methods described in WO2011/151271,the above described arrangement of the plug body means that the plug ismuch easier to drilled out that standard plug bodies which are typicallymade from single solid metal tube (e.g. steel) of greater thickness.

Preferably the reinforcement means that attach the inner and outer tubestogether comprises corrugated metal located in the gap between the innerand outer tubes.

Alternatively the reinforcement means that attach the inner and outertubes together comprises a ‘honeycomb’ metal mesh located in the gapbetween the inner and outer tubes.

A second aspect of the present invention relates to a plug assembly witha variable cross-sectional area in a plane perpendicular to the plane inwhich the assembly is deployed during the plugging of undergroundconduits (i.e. such as those suffering from drifting or otherobstacles), said assembly comprising: a plug having a plug body with afirst cross-sectional area corresponding to the minimum cross-sectionalarea of the assembly; at least one compressible plug portion that isresiliently biased to form a second cross-sectional area correspondingto the maximum cross-sectional area of the assembly; and a plugdeployment heater releasably engageable within the plug, and comprisingeutectic alloy retaining means that retain an eutectic alloy in-linewith the plug during deployment of the assembly within an undergroundconduit so as to enable the minimum cross-sectional area of the assemblyto be achieved.

When abandoning a well bridge plugs need to be placed as close aspossible to the producing zones as required by legislation (differentfrom country to country, state to state).

This can cause significant issues as the well hole or the casingsupporting the well can be damaged over the life time of the well, whichcan cause restrictions that reduce the size of the well bore.

Redundant down-hole equipment can also restrict access to the lowerregions of the well. Such equipment can sometimes be difficult orimpossible to remove due to damage, scaling or corrosion.

Bridge plugs traditionally have a small drift (i.e. the distance betweenthe outside of the tool and the inside of the well bore) this means thatthey have difficulty in by passing these restrictions so that they canbe placed in the position that may be required by the regulations.

Attempts have been made to overcome this by reducing the size of theplug relative to the well hole/casing that is to be plugged. Howeverthis has an effect on the reliability and holding pressure of the bridgeplug.

Another issue is that traditional bridge plugs have a narrow operatingwindow as two different versions might be needed for the same casingsize (i.e. 4 and half inches or about 12 cm) but different casingweights, as the drift can increase significantly as the casing weightdecreases. In older wells the records of casing weights can beincomplete so it difficult to know whether a plug will hold as theweight and hence the drift is unknown so the plug might be out of itsspecification range.

The plug assembly provided in this aspect of the present invention iscapable of reducing its cross-sectional area (e.g. diameter) as it meetswith obstacles during its delivery into the well casing and thenspringing back to an increased cross-sectional area once it has passedthe obstructions.

This ability of the assembly to return to the increased cross-sectionalarea allows any space between the plug and a well casing to beminimized, thus facilitating the formation of an effective bismuth plug.

Furthermore, by delivering the eutectic alloy into the well casingin-line with the plug the cross-sectional area can be kept to a minimum,whereas in the past the alloy might have been located on the outside ofthe plug increasing its diameter. Preferably the at least onecompressible plug portion comprises an umbrella spring arrangement whichis expandable to increase the cross-sectional area of the assembly andcompressible to decrease the cross-sectional area of the assembly.

Further preferably the umbrella spring arrangement is formed from orcoated in a material capable of withstanding high temperatures. In thisway umbrella spring arrangement can make contact with the walls of thewell casing to retain the molten alloy in close proximity with the plugduring cooling.

Preferably the alloy retaining means comprise a dump bailer located onthe same axis to and in-line with the plug. Further preferably the dumpbailer comprises release means that can be operated remotely todischarge the alloy into the area adjacent to the plug.

Advantageously the dump bailer and the ignition means of the plugdeployment heater may be triggered in a two-stage process by way of asingle ‘go’ signal received by the assembly. In one arrangement thefirst stage involves the dump bailer release means being activated todischarge the alloy and then the second stage involves the activation ofthe heater.

It is envisaged, however, that this triggering series may advantageouslybe reversed so that the heater has already started to heat up when thealloy is released.

Although the dump bailer is described above in combination with the restof the deployment apparatus it is appreciated that the in-line dumpbailer may be used on a deployment apparatus (e.g. heater) that does notemploy the variable cross-sectional area capability. The presentinvention therefore provides a plug deployment apparatus comprising anin-line dump bailer.

It is envisaged that Germanium/bismuth alloys and the thermite heatingcompositions may be used in combination with any of the other aspects ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present invention ill now be described withreference to the drawings, wherein:

FIG. 1 shows a cut away side view of a plug according to the firstaspect of the present invention;

FIG. 2 shows an end view of a plug according to the first aspect of thepresent invention;

FIG. 2A shows an end view of the plug of FIG. 2 having a honeycomb meshreinforcement.

FIG. 3 shows an exposed view of the plug assembly of the second aspectof the present invention;

FIG. 4 shows the plug assembly of FIG. 3 in situ within a well casing.

DESCRIPTION OF THE VARIOUS ASPECTS OF THE PRESENT INVENTION

It is envisaged that the various aspects of the present invention can beuse alone or in combination with one another to provide real benefits inthe plugging of underground conduits. In this regard it is envisagedthat the present invention is particularly applicable in plugging bothvertical and non-vertical wells (with or without well casings).

The described aspects can also be used together with the methods andapparatus described in WO2011/151271 to facilitated the squeezing offand repairing of wells.

Whilst the various aspects of the present invention are consideredparticularly applicable to the plugging of oil and gas wells it isenvisaged that they would provide benefits when plugging other forms ofunderground conduits such as water pipes for example.

FIGS. 1 and 2 show a plug according to the first aspect of the presentinvention. The plug is shown in a simplified view for the sake ofclarity, however it should be assumed that other features (e.g. heaterengaging means) required of plugs of the type described in this documentand in WO2011/151271 can be adopted without departing from the conceptof this aspect of the present invention.

FIG. 1 shows a cut-away side view of the plug 1 from which the outertube 2 and the inner tube 3 can be appreciated. The inner and outertubes, which are preferably circular in cross-section but may have othershapes as required, are spaced apart by the reinforcement means 4 (notshown in FIG. 1). The plug 1 has a central heater receiving void 7located along the axis of the plug to enable a plug deployment heater tobe received therein.

So that the plug has the required structural strength and resilienceboth the inner and outer tubes are made from a metal, preferably carbonsteel, stainless steel or titanium or other metal alloys suitable forthe down-hole conditions. The thickness of the tube walls is in theregion of 2 to 3 mm, although wall thicknesses in the region of 1 mm arealso contemplated. The two tubes would normally be of a similarthickness.

The plug 1 is provided with a base 5 to which both the inner and outertubes are connected, for example by welding. The feature is preferablein most embodiments of this aspect of the present invention.

Although spaced apart, the inner 2 and outer 3 tubes are connectedtogether to form a structurally sound plug by virtue of reinforcementmeans 4. The reinforcement means 4 take the form of corrugated metal(preferably steel or titanium) which is bent back and forth between theinner wall of the outer tube 2 and the outer wall of the inner tube 3.

Wherever the reinforcement means 4 touch the walls of the tubes there isa connection. Preferably the connection is formed by welding or amechanical fixing (e.g. bolts).

The reinforcement means 4 serves to hold the inner and outer tubestogether in such a way as to form a plug with the required level ofstructural strength. However it is envisaged that by replacing a singlesolid tube having a thick metal wall—as is currently used in plugbodies—with two tubes with thinner metal walls makes it much easier todrill through the plug body. This provides a further option for removingan unwanted well plug when alternative methods of extraction are notpossible.

It is envisaged that alternative forms of reinforcement means could beadopted between the inner and outer tubes to provide the same benefitsas the corrugated metal. Another possible example is considered to be ametal honeycomb mesh 4a of FIG. 2A.

FIGS. 3 and 4 relate to the plug deployment assembly 10 of the secondaspect of the present invention. The assembly 10 shown in the figurescomprises a plug body 11 (such as, but not necessarily, the onedescribed above), a heater 12 and an igniter wire 13.

The plug body 11 is provided with an umbrella spring arrangement 14which is mounted to the leading end of the plug 11 so that, when theassembly 10 is delivered down an underground conduit (such as a wellcasing) it is the umbrella spring arrangement 14 that leads the way.

The umbrella spring arrangement 14 is resiliently biased to an expandedstate, as shown in the figures. In its expanded state the umbrellaspring arrangement 14 serves to increase the effective width of the plug11 and the assembly 10.

However when the umbrella spring arrangement 14 meets with obstructionsas it is delivered down a well it has the capability to compress,thereby enabling the effective width of the assembly to be minimized tothe size of the plug body. Once past the obstruction the umbrella springarrangement springs back to its expanded state.

This arrangement enables the assembly 10 the present invention to benavigated down wells and other underground conduits that may besuffering from issues such as collapsed casing or may have other formsof obstacle (i.e. abandoned equipment) in them. It is also commonpractice to have to set plugs through production tubing to enable to getto the desired location; this also involves using a plug that can expandonce it is through the tubing. This enables plugs to be deployed intowells in situations that previously might have been impossible, or atbest a costly exercise.

The heater 12 is releasably engaged within the plug body 11 so that theheater can be retrieved from the plug body once it has been fixed into awell and the eutectic alloy plug has formed.

The heater 12, which is preferably a thermite based chemical reactionsource heater, is provided with a heater core 15 and anigniter/initiator 16. The heater 12 is attached to the igniter wire 13so that the assembly can be delivered down a well and then the heatercan be subsequently retrieved. The igniter wire 13 is connected to astandard wireline connector 17 to facilitate the delivery of theassembly down a well.

The igniter wire 13 is connected to the igniter/initiator 16 through thewireline connector 17 to enable the remote operation of the initiator 16from ground level. Preferably, and as mentioned above, a twin stageactivation of both the igniter and the dump bailer may be achieved by asingle signal.

It is envisaged that the igniter/initiator 16 might be alternativelyinitiated by a pressure pulse, radio wave, fiber optic cable, timers orother remote means.

This enables the heater to be deployed using coiled tubing or even slickline, which are non-electrically conductive delivery mechanisms). Thisis particularly useful when using heaters with chemical source heatersrather than electrical heat sources, which require a constant supply ofelectrical current to power them.

The assembly 10 is further provided with a dump bailer 18, within whichthe eutectic alloy (e.g. bismuth alloy) is transported down the well tothe site where the plug is to be formed.

In order to minimize the effective width of the assembly, and aid itsdelivery down a well, the dump bailer is located above but in-line withthe heater/plug in the region adjacent to the wireline 13.

The alloy is preferably provided in the form of shot or small beads 19so that it can freely escape the dump bailer 18 via the release means20. As with the initiator 16, the release means 20 can be operatedremotely via the wireline connector 17 or other means (see above).

Once the release means 20 are actuated gravity ensures that the alloyshot is ejected from the dump bailer into the region adjacent to theplug 11 and the heater 12. However it is envisaged that alternativemeans for ejecting the alloy from the dump bailed might be adoptedwithout departing from the general concept of the invention.

As will be appreciated from FIG. 4, the expanded umbrella springarrangement 14 makes contact with the side walls of the undergroundconduit 21 so that the alloy shot 19 does not simply fall past the plug11. Once collected adjacent the plug/heater the heater can be actuatedto melt the alloy and form a molten alloy. The umbrella springarrangement 14 is preferably made from, or coated in, a heat resistantmaterial to ensure that the molten alloy does not melt through it.

The molten alloy is then allowed to cool where upon it expands to securethe plug body 11 relative to the underground conduit 21. Once the alloy19 a has cooled (and the plug is secure) the heater 12 can be extractedusing the wireline 13.

Although not essential, it is envisaged that the alloy delivered by theassembly 10 might be a Germanium/Bismuth alloy, which has a highermelting temperature than other Bismuth based alloys. The higher meltingtemperatures of such alloys make them particularly suitable for pluggingdeeper underground where the subterranean environment is hotter.

In such applications it is appreciated that a chemical heater isrequired due to the increased level of heat required to melt the alloy(e.g. 550.degree. C.). In particular it is appreciated that a chemicalreaction heat source with a fuel composition comprising a mix ofthermite and a damping agent would be particularly preferable, withsolid mixes of these fuel compositions being especially desirable.

What is claimed is:
 1. A plugging system for plugging wells, and inparticular oil and gas wells, said plugging system comprising a plugbody formed from an outer metal tube and an inner metal tube, whereinthe tubes are concentric; reinforcement means, attached to an innersurface of the outer tube and attached to an outer surface of the innertube, thereby connecting the inner and outer tubes and thereby providingcross-sectional structural strength to the plug body; and wherein anupper end of the plug body has a central heater receiving void locatedalong the axis of the plug, wherein the void is defined by an innersurface of the inner tube; and a lower end of the plug body having abase extending across the entire plug body and to which both the innerand outer tubes are connected.
 2. The plugging system of claim 1,wherein the reinforcement means attached to the outer tube comprisescorrugated metal.
 3. The plugging system of claim 1, wherein thereinforcement means attached to the outer tube comprises a honeycombmetal mesh.
 4. A plugging system according to claim 1, wherein the innermetal tube is co-axially with the outer metal tube; and wherein thereinforcement means connects the inner surface of the outer tube and theouter surface of the inner tube at discrete points.
 5. The pluggingsystem of claim 4, wherein the reinforcement means that attach the innerand outer tubes together comprises corrugated metal located in the gapbetween the inner and outer tubes.
 6. The plugging system of claim 4,wherein the reinforcement means that attach the inner and outer tubestogether comprises a honeycomb metal mesh located in the gap between theinner and outer tubes.